Making alloca more safe

[Yigal Chripun]

Andrei Alexandrescu wrote:
> Denis Koroskin wrote:
>> On Mon, 16 Nov 2009 19:27:41 +0300, Andrei Alexandrescu 
>> <SeeWebsiteForEmail at erdani.org> wrote:
>>
>>> bearophile wrote:
>>>> Walter Bright:
>>>>
>>>>> A person using alloca is expecting stack allocation, and that it 
>>>>> goes away after the function exits. Switching arbitrarily to the gc 
>>>>> will not be detected and may hide a programming error (asking for a 
>>>>> gigantic piece of memory is not anticipated for alloca, and could 
>>>>> be caused by an overflow or logic error in calculating its size).
>>>>  There's another solution, that I'd like to see more often used in 
>>>> Phobos: you can add another function to Phobos, let's call it 
>>>> salloca (safe alloca) that does what Denis Koroskin asks for (it's a 
>>>> very simple function).
>>>
>>> Can't be written. Try it.
>>>
>>> Andrei
>>
>> It's tricky. It can't be written *without a compiler support*, because 
>> it is considered special for a compiler (it always inlines the call to 
>> it). It could be written otherwise.
>>
>> I was thinking about proposing either an inline keyword in a language 
>> (one that would enforce function inlining, rather than suggesting it 
>> to compiler), or allways inline all the functions that make use of 
>> alloca. Without either of them, it is impossible to create wrappers 
>> around alloca (for example, one that create arrays on stack 
>> type-safely and without casts):
>>
>> T[] array_alloca(T)(size_t size) { ... }
>>
>> or one that would return GC-allocated memory when stack allocation fails:
>>
>> void* salloca(size_t size) {
>>     void* ptr = alloca(size);
>>     if (ptr is null) return (new void[size]).ptr;
>>
>>     return ptr;
>> }
> 
> The problem of salloca is that alloca's memory gets released when 
> salloca returns.
> 
> Andrei

template salloca(alias ptr, alias size) { // horrible name, btw
   ptr = alloca(size);
   if (ptr is null) ptr = (new void[size]).ptr;
}

// use:
void foo() {
   int size = 50;
   void* ptr;
   mixin salloca!(ptr, size);
   //...
}

wouldn't that work?